This invention relates to devices for controlling the heating of fuser roll apparatus, in which a toner powder image on a supporting sheet is heated under pressure, thereby being fixed thereon.
It is a common practice in dry type electrophotographic copying machines that an electrostatic charge of a specific polarity is uniformly charged over the entire surface of an electrophotographic sensitive plate, and then exposed to a light pattern of an original document to be reproduced, whereby an electrostatic latent image is formed on the sensitive plate. The electrostatic latent image thus formed is developed into a powder image with a developer called "toner" having a polarity opposite to that of the electrostatic latent image. The powder image thus developed is fixed on a copying paper, and thus a copy having a pattern conforming to that of the original document is obtained.
A fuser roll apparatus is one of the powder image fixing apparatus whose fixing performance is most rapid and efficient. In the fuser roll apparatus of the type described, the surface of a fuser roll is directly pressed on a toner image on the surface of the supporting sheet for heating the toner image, so that the performance of the apparatus is greatly influenced by the surface temperature of the fuser roll. For instance, where the fuser roll surface temperature is lower than a proper fixing temperature, or, to the contrary, where the surface temperature of the fuser roll is too high to fix toner image properly, there occurs in either case, transfer of toner powder image onto the surface of the fuser roll, leading to the so-called offset phenomenon, in which toner image on the surface of the fuser roll is transferred to successive copying paper during the subsequent reproduction cycle.
This necessarily narrows the allowable temperature range of the fuser roll surface, within which the proper fixing of a toner image may be effected.
The surface temperature of a fuser roll is not constant along the axial direction thereof, because of variation in the distribution of heat from a heater for heating the fuser roll, and because of heat discharge from the rotary shaft of the fuser roll to a bearing. If a surface temperature detecting element is provided in the mid portion of the fuser roll for controlling the surface temperature, there may result a lowering of temperature on the opposite end portions of the fuser roll down to a temperature lower than the critical surface temperature, due to the above-described reasons, with the resulting failure to achieve the proper fixing of a toner image.
Meanwhile, it is a recent trend to continuously produce a number of copies of different paper sizes, such as letters paper size, legal size, or computer size, from a single copying machine. Therefore, reproducing machines are designed to have a width conforming to a maximum copy size. In such a reproducing machine, when the fixing of a toner image on a copying sheet of a maximum size is effected, the surface temperature of a fuser roll becomes temporarily lowered uniformly over the entire surface thereof, while the initial surface temperature is then restored by heating the fuser roll by means of a heater. In contrast thereto, upon fixing of a toner image on a copying sheet having a width narrower than the maximum copy size, the surface temperature of the fuser roll becomes lowered only in the contacting area of the fuser roll with the copying sheet. With the temperature condition of the fuser roll being maintained, if the fuser roll is heated, there results an abnormal temperature rise in an area on the surface of the fuser roll, in which the temperature drop has not occurred, leading to uneven fixing in subsequent reproduction cycles.
In prior art reproducing machines, no satisfactory countermeasure has been devised for solving the aforesaid problems which is experienced with the continuous production of plural copies of different sizes.